1. Field of the Invention
The present invention relates generally to an optical wavelength converting device for providing a coherent light source required in a photo information processing field and a photo applied measuring control field and a manufacturing method of the device, and more particularly to a device for converting fundamental waves into second harmonic waves to generate shorter wavelength light and a manufacturing method of the device. Also, the present invention relates to a shorter wavelength coherent light generating apparatus with the device.
2. Description of the Related Art
A ferroelectric substance having a plurality of inverted-polarization layers periodically arranged is formed by forcibly inverting spontaneous polarization of the ferroelectric substance. The ferroelectric substance having the inverted-polarization layers have been utilized as an optical frequency modulator utilizing surface acoustic waves and an optical wavelength converting device utilizing non-linear polarization inversion of a non-linear optic substance. Particularly, in cases where non-linear polarization of the non-linear optic substance is periodically inverted to produce alternate rows of non-linear polarization layers and non-linear inverted-polarization layers, fundamental waves can be efficiently converted into second harmonic waves by transmitting the fundamental waves through the alternate rows. Therefore, a small-sized shorter wavelength coherent light generating apparatus can be manufactured by combining a semiconductor laser and the non-linear optical substance to transmit coherent light radiated from the semiconductor laser through the alternate rows of the non-linear optical substance. Because the small-sized shorter wavelength coherent light generating apparatus is useful for a printing field, a optical information processing field, and an optical applied measuring control field, the research of polarization inversion in the non-linear optical substance has been enthusiastically performed.
2.1. Previously Proposed Art
A conventional manufacturing method of inverted-polarization layers periodically arranged in a LiTaO.sub.3 substrate has been proposed in Japanese Patent Application No. 301667 of 1991 which was laid open to public inspection on Feb. 5, 1993 under Provisional Publication No. 27288/93 (H5-27288). In J.P.A. 301667, a plurality of proton (H.sup.+) exchange layers are periodically arranged in a -C lattice plane of a LiTaO.sub.3 substrate according to a selective proton exchange method. Thereafter, the proton exchange layers are rapidly heated according to an infrared heating method and are changed to inverted-polarization layers. The conventional manufacturing method is described with reference to FIGS. 1A to 1E in detail.
FIGS. 1A to 1E are cross sectional views showing a conventional manufacturing method of a conventional optical wavelength converting device in which inverted-polarization layers and non-inverted polarization layers are periodically arranged in an upper side of LiTaO.sub.3 substrate, and FIGS. 1F to 1H are diagonal views showing the conventional manufacturing method.
As is well known, LiTaO.sub.3 crystal has X-, Y-, and C-crystal axes, and a spontaneous polarization of the LiTaO.sub.3 crystal is directed in a +C-crystal axis direction of the C-crystal axis.
As shown in FIG. 1A, a LiTaO.sub.3 substrate 11 having a -C lattice plane on its upper surface is prepared. The LiTaO.sub.3 substrate 11 is formed by cutting out LiTaO.sub.3 crystal in a perpendicular direction to the C-crystal axis defined as a crystal orientation [001], and the upper surface of the LiTaO.sub.3 substrate 11 is directed towards a -C-crystal axis direction. Therefore, the -C lattice plane is defined as (001) plane in Miller indices. Also, a spontaneous polarization Ps of the LiTaO.sub.3 substrate 11 is directed in a lower direction (or +C-crystal axis direction). Thereafter, Ta atoms are deposited on the upper surface region of the LiTaO.sub.3 substrate 11 according to a sputtering method to produce a Ta mask 12 at a thickness of 30 nm.
Thereafter, a periodic pattern is drawn on the Ta mask 12 according to a lithography process as shown in FIG. 1B, and the Ta mask 12 is etched in the periodic pattern according to a dry etching process as shown in FIG. 1C. Therefore, patterned Ta masks 12P are periodically arranged on the LiTaO.sub.3 substrate 11 at regular intervals.
Thereafter, as shown in FIG. 1D, the LiTaO.sub.3 substrate 11 is immersed in a pyrophosphoric acid (H.sub.4 P.sub.2 O.sub.7) solution to periodically form a plurality of proton exchange layers 13 in the LiTaO.sub.3 substrate 11 not covered with the Ta masks 12P according to a first proton exchange process. In detail, a part of Li.sup.+ ions of the LiTaO.sub.3 substrate 11 are exchanged for H.sup.+ ions of the pyrophosphoric acid solution so that the proton exchange layers 13 made of H.sub.(1-x) Li.sub.x TaO.sub.3 are periodically formed.
Thereafter, the LiTaO.sub.3 substrate 11 with the proton exchange Layers 13 is heated up at a rising rate of about 10.degree. C./second according to an infrared heating method. Thereafter, the thermal processing of the proton exchange layers 13 are continued at a temperature of 450 .degree. C. Therefore, as shown in FIG. 1E, the H.sup.+ ions in the proton exchange layers 13 are thermally diffused into the LiTaO.sub.3 substrate 11 at a prescribed thermal diffusion speed, and the spontaneous polarization Ps directed in the lower direction is inverted to an upper direction in a region thermally diffused by the H.sup.+ ions to form inverted-polarization layers 14. Thereafter, the Ta masks 12P is taken off as shown in FIG. 1F.
Thereafter, Ta material is deposited on the LiTaO.sub.3 substrate 11 with the inverted-polarization layers 14 and is patterned to form a Ta film 15 having a slit 15A, as shown in FIG. 1G. Thereafter, the LiTaO.sub.3 substrate 11 with the inverted-polarization layers 14 is immersed in the pyrophosphoric acid solution according to a second proton exchange process to change the LiTaO.sub.3 substrate 11 and the inverted-polarization layers 14 not covered with the Ta film 15 to a high refractive index layer. Thereafter, the Ta film 15 is taken off, and the high refractive index layer is annealed. As a result, as shown in FIG. 1H, the high refractive index layer is changed to an optical waveguide 16 which is composed of alternate rows of the inverted-polarization layers 14 and non-inverted polarization layers 17 processed according to the second proton exchange process, and the manufacturing of a conventional optical wavelength converting device 18 is finished.
Next, a forming mechanism of the inverted-polarization layer 14 in the LiTaO.sub.3 substrate 11 having the -C lattice plane is described to simplify the apprehension of the present invention. The forming mechanism is made clear by inventors of the present invention and is classified to two steps. A first step of the forming mechanism is that an inverted-polarization kernel functioning as a seed of the inverted-polarization layer 14 is formed because an internal electric field is induced in an upper surface region of the LiTaO.sub.3 substrate 11. A second step of the forming mechanism is that the inverted-polarization kernel is growing large to form the inverted-polarization layer 14.
The forming mechanism relating to the inducement of the internal electric field is described in detail with reference to FIG. 2.
When the LiTaO.sub.3 substrate 11 is immersed in the pyrophosphoric acid (H.sub.4 P.sub.2 O.sub.7) solution, Li.sup.+ ions existing in +C and -C surface regions of the LiTaO.sub.3 substrate 11 are exchanged for H.sup.+ ions of the pyrophosphoric acid solution. Therefore, as shown in FIG. 2, an upper proton exchange layer 13A composed of H.sub.(1-x) Li.sub.x TaO.sub.3 is formed in the -C surface region of the LiTaO.sub.3 substrate 11, and a lower proton exchange layer 13B composed of H.sub.(1-x) Li.sub.x TaO.sub.3 is formed in the +C surface region of the LiTaO.sub.3 substrate 11. Thereafter, when the LiTaO.sub.3 substrate 11 is heated at high temperature, the H.sup.+ ions densified in the proton exchange layers 13A, 13B are thermally diffused into an internal portion of the LiTaO.sub.3 substrate 11. Also, the Li.sup.+ ions densified in the internal portion of the LiTaO.sub.3 substrate 11 are thermally diffused into the proton exchange layers 13A, 13B. However, because the thermal diffusion speed of the H.sup.+ ions is faster than that of the Li.sup.+ ions, the proton exchange layers 13A, 13B are charged with negative electricity, and the internal portion of the LiTaO.sub.3 substrate 11 is charged with positive electricity. Therefore, a first internal electric field E.sub.1 directed in the -C-crystal axis direction is induced in a first boundary region between the first proton exchange layer 13A and the internal portion of the LiTaO.sub.3 substrate 11, and a second internal electric field E.sub.2 directed in the +C-crystal axis direction is induced in a second boundary region between the proton exchange layer 13B and the internal portion of the LiTaO.sub.3 substrate 11.
In this case, because the direction of the second internal electric field E.sub.2 is the same as that of the spontaneous polarization Ps of the LiTaO.sub.3 substrate 11, the polarization of the second boundary region remain directed in the direction of the spontaneous polarization Ps. In contrast, because the direction of the first internal electric field E.sub.1 is opposite to that of the spontaneous polarization Ps of the LiTaO.sub.3 substrate 11, the polarization direction in the first boundary region is inverted so that an inverted-polarization kernel 19 is generated in the first boundary region. The direction of the polarization of the inverted-polarization kernel 19 is opposite to that of the spontaneous polarization Ps of the LiTaO.sub.3 substrate 11. Thereafter, the inverted-polarization kernel 19 is growing large while the LiTaO.sub.3 substrate 11 is heated at high temperature. Accordingly, the inverted-polarization layer 14 is produced in the upper surface region of the LiTaO.sub.3 substrate 11.